The invention relates to Faraday-effect type current sensors, and more particularly to a sensor coil with improved immunity to mechanical effects, in particular rotation and vibration of the coil.
Fiber-optic devices are attractive for sensing a magnetic field induced by an electric current, in particular if the electric current is carried by wires having a substantial electric potential with respect to ground. Such fiber-optic current sensors can be made quite small and can be constructed to withstand considerable mechanical shock, temperature change, and other environmental extremes. Due to the absence of moving parts, they can be nearly maintenance free, and they have the potential of becoming economical in cost.
A fiber-optic current sensor has an optical fiber wound in the form of a coil which surrounds the current-carrying wire of which the current is to be sensed. The coil may have one turn to several hundred turns, and is part of a closed optical path in which an electromagnetic wave, such as a light wave, is introduced. The light is typically circularly polarized with opposite polarization directions, with the opposite polarization directions forming counter-propagating waves which traverse the coil in clockwise (cw) and counterclockwise (ccw) directions. The counter-propagating waves are then recombined and impinge on a photodetector. As a result of the magnetic-field induced Faraday effect, the optical rotation increases in one polarization direction (for example, cw) and decreases in the other polarization direction (in this example, ccw). The opposite result occurs for current flow in the other direction. The difference in the optical rotation between the counter-propagating waves introduces a phase shift between these waves, which is known as the Faraday effect.
In general, there may be reasons why the fiber-optic current sensor does not give the expected current sensing accuracy. One of these is due to the presence of vibration or angular rotation. Vibrations can produce a phase shift via the rotational motion of the sensing coil (the Sagnac effect). The sensor is, in effect, both a gyro and a current sensor; responding to angular rotation as well as to magnetic field. For example, a sensing coil having a diameter of 137 mm and 4 turns of fiber has a Verdet constant of 2.5xc3x9710xe2x88x926 radians/Ampere-turn, the optical phase shift produced by an angular rotation of the coil of 1xc2x0/sec is equivalent to that produced by a current of 100 Amps. Such a rotation would be produced by an azimuth displacement of 240 xcexcm at the outer diameter of the coil at a vibrational frequency of 50 Hz.
Another cause of optical phase difference shift xcex4 at angular frequency xcfx89v due to vibration is that of an actual angular vibration which induces a true AC rotation rate. This effect takes the same functional form for a vibration-induced phase difference modulation
xcex4=xcex94"psgr"vcos(xcfx89vt+xcex5).
The output of the current sensor will correctly indicate the actual magnetic field environment, but the output will vary at xcfx89v.
It would therefore be desirable to provide a current sensor which is only slightly or not at all affected by vibrations in the current sensor coil.
According to one aspect of the invention, the effect arising from sensing coil vibrations and rotation can be substantially reduced or even eliminated by forming a fiber-optic current sensor coil of a fiber sensing coil section made of a first fiber that has substantially no birefringence and is wound in a first winding direction about an axis; and at least one xe2x80x9cbuckingxe2x80x9d coil made of a second fiber that has a large birefringence and is wound in a second winding direction about the same axis. The first fiber and the second fiber of the at least one fiber bucking coil section are connected to one another so that optical radiation, when viewed along the axis, propagates through the first fiber in a direction opposite to the direction of the light propagating through the second fiber. The at least one fiber bucking coil section is insensitive to the Faraday effect and has substantially the same effective total area as the current sensing coil section.
The bucking coil is advantageously placed adjacent to the current sensing coil and is bonded to it so that both coils experience substantially identical rotation, acceleration or vibration. The first and second fiber can also be placed next to each other during the winding operation or bonded to one another before the winding operation. The bucking coil can be a highly birefringent fiber, such as an elliptically cored fiber, which is insensitive to the Faraday effect. At least one first quarter wave plate can be disposed in a region where the first fiber of the current sensing coil section and the second fiber of the at least one fiber bucking coil section are connected to one another.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.